screen: Make meta_screen_foreach_window scan Wayland windows

Scanning over the hash table of XIDs is a terrible idea. Not only were
we excluding Wayland windows, but we were also looking at alarms and
barriers, too. We were lucky that that only contained GObjects where
our checks would work.
screen: Drop the MetaScreen parameter from the foreach function

It's unused.
screen: Add the list window flags to meta_screen_foreach_window

So we can ditch a manual use of list_windows.
prefs: Actually insert the SCHEMA_MOUSE settings